Update: Make the preferences window content scrollable

Previously it could happen that the preferences window got larger than
the height of the screen (e.g. when having a bunch of items and a low
vertical screen resolution).
Fix this, by making the preferences window content scrollable.

Also introduce a nice default window size.

src/prefs-widget.ui

@@ -18,7 +18,9 @@
   along with this program.  If not, see <https://www.gnu.org/licenses/>.
 -->
 <interface>
-    <template class="PrefsWidget" parent="GtkBox">
+    <template class="PrefsWidget" parent="GtkScrolledWindow">
+        <child>
+            <object class="GtkBox">
                 <property name="orientation">vertical</property>
                 <property name="spacing">24</property>
                 <property name="margin-top">36</property>
@@ -97,5 +99,7 @@
                         </child>
                     </object>
                 </child>
+            </object>
+        </child>
     </template>
 </interface>

src/prefs.js

@@ -36,12 +36,33 @@ var PrefsWidget = GObject.registerClass({
         "center-box-order",
         "right-box-order"
     ]
-}, class PrefsWidget extends Gtk.Box {
+}, class PrefsWidget extends Gtk.ScrolledWindow {
     _init(params = {}) {
         super._init(params);
 
         this._settings = ExtensionUtils.getSettings();
 
+        // Never show a horizontal scrollbar.
+        // Achieved by setting the hscrollbar_policy to 2, while setting the
+        // vscrollbar_policy to 1 (the default value).
+        this.set_policy(2, 1);
+
+        // Set the default size of the preferences window to a sensible value on
+        // realize.
+        this.connect("realize", () => {
+            // Get the window.
+            const window = this.get_root();
+
+            // Use 500 and 750 for the default size.
+            // Those are the same values the Just Perfection Gnome Shell
+            // extension uses.
+            // It seems like those values only get used the first time the
+            // preferences window gets opened in a session. On all consecutive
+            // opens, the window is a bit larger than those values.
+            window.default_width = 500;
+            window.default_height = 750;
+        });
+
         // Initialize the given `gtkListBox`.
         const initializeGtkListBox = (boxOrder, gtkListBox) => {
             // Add the items of the given configured box order as